In the penultimate paragraph of perhaps the second-greatest speech ever delivered on American soil, Abraham Lincoln set forth the reason the nation was engaged in a great Civil War.

“One-eighth of the whole population were colored slaves, not distributed generally over the Union, but localized in the Southern part of it,” Lincoln said in his second inaugural address. “These slaves constituted a peculiar and powerful interest. All knew that this interest was somehow the cause of the war.”

That’s pretty clear: The Civil War was fought because of slavery. So if that were so evident at noon on March 4, 1865, why has there been such controversy for a century and a half over the real cause of the Civil War?

That is a far more difficult question. For reasons economic, social, political and historical, both sides in the Civil War have portrayed the struggle as one over states’ rights, or the size of government or the economic destiny of the country. Slavery was the cause that dared not speak its name.

These other issues were factors, to be sure. But they all grew out of slavery and the great divide that slavery created — first between black and white, then between landowner and laborer, next between North and South, finally between the Union and the Confederacy.

Slavery was central

Strains of these struggles and these divides still mark us. Perhaps they will persist longer than Lincoln’s prescription, when he suggested later in that splendid inauguration paragraph that the fighting would last “until all the wealth piled by the bondsman’s 250 years of unrequited toil shall be sunk, and until every drop of blood drawn with the lash shall be paid by another drawn with the sword.”

But we cannot erase those divisions until we face, forthrightly and courageously, the issue that seems so obvious 150 years later and that still seems so difficult to confront. Slavery, and not economic and political independence, was at the center of the war. To those who deny it, consider this question: Would there have been a civil war on American soil if for some reason slavery were not planted on this continent along with tobacco and cotton?

“When you talk to people around the country and ask them the causes of the Civil War, you get the same answers you would have gotten in 1861,” says Andrew E. Masich, president of Pittsburgh’s Heinz History Center. “Some white people in the South think it was a war to protect their homeland and states’ rights. Some blacks can’t believe the war was about slavery because they don’t believe white people would fight for them. And in the North there are the Unionists, who think the war was fought to preserve the country. There’s still no agreement on the root causes of the Civil War.”

Multiple causes?

In truth, it is possible to search the remarks of Abraham Lincoln and to find in them multiple causes of the Civil War. He began his presidency hoping to restrict the spread of slavery. Then he struggled to preserve the Union. Only later did he decide the fight was about freedom. And by freedom, he meant freedom for all, a broad freedom as envisioned in the Declaration of Independence but — and here is where Lincoln charted new territory — applied to all Americans, white and black.

In the greatest speech ever delivered on American soil, Lincoln spoke of “unfinished work,” and he was not talking only about the task of winning the war. He was speaking, too, of winning that broader definition of freedom. That is what he meant by the phrase “new birth of freedom.” And his opening, speaking of what happened fourscore and seven years earlier — in 1776 — makes it clear that he is taking the Declaration of Independence and grafting it onto the American Constitution, or at least onto the American conscience, with its timeless celebration of the Enlightenment principle that “all men are created equal.”

But even the American Constitution was marked by slavery. The specter haunting the Constitutional Convention, along with the fecklessness of the doomed Articles of Confederation, was slavery. The founding fathers did many things, but they punted on slavery, leaving it to a future generation to settle. That future generation dealt with it by fighting over the Missouri Compromise and the Kansas-Nebraska Act and then fighting the Civil War.

“Race was a central factor in the nation until the Civil War, through the Civil War and beyond the Civil War,” Rep. Jesse Jackson Jr., the Illinois Democrat who has proposed a national commission to commemorate the 150th anniversary of the Civil War, said in an interview. “There never has been a moment that slavery and race was not a factor, and yet both sides were able to subvert the issue of race and not deal with the role of slavery in the war.”

Facing reality

During the war, the primacy of slavery was incontrovertible. In its commentary on the firing on Fort Sumter, the Chicago Tribune said “that barbarous institution” was “the cause of the rebellion which months of effort has ripened into the bloody strife this day commenced.” Frederick Douglass said at the war’s start that the cessation of slavery should be the price of secession from the Union.

We have to face this issue not only to get our past right. We also have to answer it to get our future right. “The past,” Harvard historian Niall Ferguson has written, “is our only reliable guide to the present and to the multiple futures that lie before us, only one of which will actually happen.”

Understand slavery and you understand the Civil War. Understand the Civil War and you understand America. The New Freedom, the New Deal, the great prosperity that followed World War II, the youth and women’s movements of the 1960s, the Reagan Revolution — they’re all important. But not one of them defines us the way the Civil War did, and does. That’s why the 150th anniversary of the Civil War is not only a commemoration. It also is an opportunity.